Nuclear Fusion Explained

Main-sequence stars are powered by the proton-proton chain (in smaller stars) or the CNO cycle (in more massive stars). Four hydrogen nuclei fuse step by step into one helium nucleus, which weighs slightly less than the four it came from. That tiny missing mass becomes energy through E = mc².

The Sun fuses about 600 million tons of hydrogen each second. Roughly 4 million tons of that vanishes as pure energy, the sunlight and heat that power our planet, making fusion far more efficient than chemical reactions like burning coal.